A seat belt apparatus equipped on a vehicle restrains an occupant on a vehicle seat with a seat belt extracted from a seat belt retractor inside a seatback to protect the occupant at the time of vehicle collision or the like. In the seat belt retractor, when an acceleration exceeding a certain value acts in a horizontal direction at the time of vehicle collision or the like, an acceleration sensor detects the acceleration to activate a seat belt lock mechanism, thereby disabling the extraction of the seat belt. As an inertia body used in the acceleration sensor, there is known a ball being used (see Patent Document 1) and a self-standing inertia body being used (see Patent Document 2).
While the lock mechanism is activated in response to an acceleration acting in the horizontal direction being detected by the acceleration sensor in the seat belt retractor, the sensitivity of the acceleration sensor by which it activates the lock mechanism (the lock sensitivity) sometimes varies, depending on a direction of an acceleration acting on the vehicle. For example, a retractor built into a seatback of a vehicle is configured such that an acceleration sensor swings in accordance with a reclining angle of the seatback. Therefore, the lock sensitivity sometimes differs between a case in which an acceleration is generated in the swinging direction and a case in which an acceleration is generated in a direction orthogonal to the swinging direction. In addition, the lock sensitivity differs depending on the direction in which the acceleration acts, also in accordance with various conditions such as a shape and a non-locking angle of a lever (an actuating member) which is interposed between the acceleration sensor and the lock mechanism.
Because of this, if the specification of the acceleration sensor is determined uniformly based on a sensitivity in a direction in which the lock sensitivity becomes the lowest so that the sensitivity becomes sharp, a lock sensitivity in a direction orthogonal to the direction in which the lock sensitivity is low becomes too sharp, resulting in a fear that an early locking is caused in a non-locking performance.
Conventionally, in order to enable an adjustment of the lock sensitivity for each direction in which acceleration acts, there is known the acceleration sensor described in Patent Document 1 in which the ball type inertia body is used and an inclination angle of a ball receiving surface is made to differ in the each direction.    Patent Document 1: JP 10-157569 A    Patent Document 2: JP 2002-321595 A
Meanwhile, in the acceleration sensor described in Patent Document 1 which uses the ball type inertia body, not only the inclination angle of the ball receiving surface but also an inclination angle of the lever needs to be set. In the acceleration sensor described in Patent Document 2 which uses the self-standing inertia body, no particular consideration is made for an adjustment depending on a direction in which an acceleration acts.